A free electron laser needs a source of electrons with high current and low emittance. Cathodes using the photoelectric effect are recognized as superior for these ends compared to cathodes using thermal emission. Development of photocathodes suitable for free electron lasers has tended to concentrate on cathode materials that emit efficiently with visible light. This requires complex cathode materials that tend to be incompatible with accelerators which use high voltage gradients and also tend to be short lived. The alternative is to use ultraviolet light to achieve photoemission from simple cathode materials. High photoelectric efficiency is well known from common metals such as copper, molybdenum, vanadium, tantalum and tungsten when irradiated by ultraviolet light with photon energy of approximately 12 electron volts or more. The limiting factor in producing enough electrons has been the intensity of the ultraviolet light. No sources of ultraviolet light commonly available could produce enough electrons to meet the requirements of a high power free electron laser.
A 40 watt source of deep ultraviolet light can generate enough current from a common metal cathode to pump a high power free electron laser.